You'd pretty much have to either do slow negatives or pause in order to make 75% of a 15 rep max feel like a workout. A 15 rep max means 14 is hard, and it's all you can do to get that 15th. Now take 75% of that weight and do the same thing. It would be WAY easier for EVERYONE. So you would have to throw something into the mix to make the body strain to complete the set, right?
Has anyone started this without the full week to 2 weeks off from the gym? I was looking to start w/ 4 days of rest in between, so on the 5th day.
A few things:
1) 15s are primarily to increase oxidative stress which has been shown to have very positive effects on tendon strengthening and repair. This can help prep you for the heavy loading to come later in the cycle.
2) If sufficiently deconditioned at the start of a cycle, the muscle tissue will benefit from a hypertrophic response to lighter loading but it will not be effective for long, hence the reason for incrementing loads each session.
3) Fatigue and the feeling that a load is hard to lift are not good ways to measure whether a certain load has been an effective stimulus for hypertrophy. ie. just because you are straining to lift a weight does not guarantee that you have created a growth stimulus. It all depends on your conditioning to the load at the time.
4) If you can't get a good burn going by slowing down rep speed with 75% of your 15RM then do more reps at a faster speed. I often do 25 reps for a set at the start of 15s. No need to go to failure though.
5) Any subsequent sets will feel harder because you will be carrying some fatigue from the previous set. During the second week of each mesocycle you may need to stop short of the intended rep target in order to stay away from failure.
6) Just try it and see! Take a proper SD - 4 days doesn't cut it. If you absolutely can't wait then up to you but 9 days is better, especially if you have been training long and hard for a few months. I usually take 9 days which means finishing up after a Friday session and then taking the whole of the next week off, to return on the following Monday.
As an example: if my 15RM for deads is 350lb, then I'm going to start off with 260lb for sets across. By the time I get to the end of my second set of 20 reps, I'm feeling fairly convinced that I've done enough deads for the night.

However many cycles I do, I am always surprised now much weaker I feel after a 9 day SD. The first light session is a very welcome way to get started again and strength picks up again quickly which is a good thing because the loads seem to get heavier quickly too!